Infrared face detection and recognition system

ABSTRACT

A face detection and recognition system having several arrays imaging a scene at different bands of the infrared spectrum. The system may use weighted subtracting and thresholding to distinguish human skin in a sensed image. A feature selector may locate a face in the image. The face may be framed or the image cropped with a frame or border to incorporate essentially only the face. The border may be superimposed on an image direct from an imaging array. A sub-image containing the face may be extracted from within the border and compared with a database of face information to attain recognition of the face. A level of recognition of the face may be established. Infrared lighting may be used as needed to illuminate the scene.

BACKGROUND

The present invention pertains to detection of people and particularly to recognition of people. More particularly, the invention pertains to detection and recognition of people under dark conditions.

The invention may be related to a patent application entitled “Infrared and Visible Fusion Face Recognition System”, having attorney docket number H0006826-760 (1100.1317101), and filed approximately concurrently with the present application.

There are many commercial off the shelf (COTS) face recognition systems (FRS) available for use. All of these systems may require strict face positioning in front of a camera and good illumination. Such conditions are often not available where face recognition may be needed or desired. In many practical situations, the subject being sought for detection and recognition may be moving while passing in front of a camera used for such tasks. Further, the illumination of the subject may be inadequate or inconsistent, or there may be total darkness.

Art that may be relevant includes U.S. Pat. No. 6,370,260 B1, issued Apr. 9, 2002, entitled “Near-IR Human Detector”, and by Pavlidis et al.; U.S. Pat. No. 6,718,049 B2, issued Apr. 6, 2004, entitled “Near-Infrared Disguise Detection”, and by Pavlidis et al.; and U.S. patent application Ser. No. 10/077,672, filed Feb. 15, 2002, entitled “Near-Infrared Method and System for Use in Face Detection”, and by Pavlidis et al.; all of which are incorporated herein by reference. The assignee of these patent documents is the same assignee of the present invention.

SUMMARY

The invention may utilize two bands of infrared light for detection and recognition of people's faces. The system may use a camera having multi-band detection capabilities. As the environment in which one wants to do recognition becomes darker, invisible lighting may be used for detection and recognition, even in complete darkness. Also, there may be facial feature identification for framing a detected face for recognition.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 reveals a two-band detection, fusion, recognition, and confidence level indication system;

FIG. 2 is an illustrative example a layout for the system of FIG. 1;

FIG. 3 shows a two-band infrared fusion system for detecting humans;

FIG. 4 illustrates a co-registration scheme for two cameras or arrays;

FIG. 5 is a histogram of the number of pixels versus spectral power for an image frame;

FIG. 6 is the graph of the electromagnetic spectrum;

FIG. 7 reveals the infrared transmittance characteristics for an automobile windshield;

FIG. 8 reveals the infrared transmittance characteristics for an automobile side window;

FIG. 9 is a graph showing the percentage of reflectance for Caucasian males of light and dark complexions;

FIG. 10 is a graph showing the percentage of reflectance for Asian males of light and dark complexions;

FIG. 11 is a graph showing the percentage of reflectance for black males of light and dark complexions;

FIG. 12 is a graph showing a comparison of reflectances for light and dark skin;

FIG. 13 is a graph of reflectances for cotton, wood and polyamide;

FIG. 14 is a graph of reflectance for distilled water;

FIG. 15 shows a layout for determining the speed characteristics of a human detection system;

FIG. 16 shows a general illustrative example of an infrared system for identifying humans using face detection and face recognition;

FIG. 17 shows an infrared identification system including a face detection process that can be implemented by the system of FIG. 16 for detecting faces of humans;

FIG. 18 shows an illustration for use in describing the possible orientation of a human head; which orientation can be at least partially estimated;

FIG. 19 shows an illustrative example of a system for implementing at least face detection;

FIG. 20 shows one illustrative example of a face detection system illustrated generally in FIG. 17;

FIG. 21 is an illustrative graph showing the control of an infrared illumination source;

FIG. 22 is an illustrative example of a system for further defining the facial search area as shown generally in FIG. 20;

FIGS. 23 a-23 h are illustrations describing a system for defining the facial search area as shown generally in FIG. 20, and more specifically in FIG. 22;

FIG. 24 is an illustrative example of an eye location system shown generally in FIG. 20;

FIG. 25 is an illustrative example of a search method for locating eye containing regions in the eye location method shown generally in FIG. 24;

FIG. 26 is one illustrative example of an approximation method for determining the approximate center of one or both eyes of a person in the eye location method shown generally in FIG. 24;

FIGS. 27 a-27 d are illustrations for use in describing a system for extracting eye and eyebrow features in the eye location system shown generally in FIG. 24;

FIG. 28 is an illustration of a template for use in the search method for locating eye containing regions in the eye location system shown generally in FIG. 24, and also further described with reference to FIG. 25; and

FIGS. 29 a-29 c are illustrations for use in describing a system for determining the approximate center of one or both eyes of a person in the eye location system shown generally in FIG. 24, and also further described with reference to FIG. 26.

DESCRIPTION

Most face recognition systems may operate in the visible spectrum (i.e., 250-700 nanometers) and thus require adequate and stable illumination on the face. Some IR-based systems may operate very close to the visible spectrum (i.e., near IR-800-900 nanometers). This appears to be too narrow of a band and also too close and similar to the visible spectrum. Thus, this band does not necessarily add much more information than the visible band does about the face.

The present invention may utilize the entire lower reflective infrared (LRIR) band (i.e., approximately 800-1400 nanometers). There may be a significant amount of reflected energy well above 800 nanometers all of the way up to 1400 nanometers. This approach may gather much more energy and information about the face over a broader band. The LRIR may be detected with a multi-band imaging (MBI) camera sensitive to that band for the enrollment, testing and matching of the detected face for recognition. The other band of interest may be the upper reflective infrared (URIR) band (i.e., approximately 1400-2200 nanometers). A LRIR sensed image may provide an image that resembles a black and white picture.

A URIR sensed image may provide an image that may distinctively show human skin/flesh with a contrast to other materials. The URIR sensed image is not like the LRIR sensed image in that the flesh-like matter does not reflect the IR in the same manner as the other matter, e.g., inanimate objects. Since the lower and upper IR bands are reflective, the scene may need a source of illumination. However, such illumination need not be visible and so will not be distraction to or seen by a human being, and may be safe to the human eyes. There may be scene detection in pure darkness. In the daytime, the LRIR and URIR illumination may be unnecessary because sufficient illumination may be provided by ordinary daylight.

The invention may work with any COTS FRS. The camera may have an integrated IR light source which is invisible to the human eye. The energy level of the source may be computer controlled in view of the image and ambient lighting conditions. Under good illumination conditions, the IR source does not have to output energy. The less ambient illumination there is, the light source may output more IR light so that a constant image intensity is maintained at the lower reflective IR band. Since one may be able to obtain a stable face image (picture) in the LRIR band, under various ambient illumination conditions, and that this IR picture contains virtually all of the structural information of the face, then the usual video image of the face, which is input to the FRS, may be replaced by the LRIR image of the face.

There may be a utilization of a procedure of enrollment and testing for a match required by any FRS. Enrollment may be performed by capturing the face of the person using the LRIR of the MBI camera, or a single band IR camera operating in the LRIR wavelength band coupled with the above-mentioned IR source. Testing for the match may be performed by using the same IR band camera coupled with the IR source. Since the gain of the output energy of the IR source may be automatically adjusted by a computer and appropriate software, the enrollment and testing for the match and face recognition may be performed under all illumination conditions, including complete darkness of the visible spectrum.

In FIG. 1, shows an image 11 that is sensed in the LRIR band. The picture may resemble a normal photograph having the black and white characteristics with a gray scale presentation. FIG. 1 further shows an image 12 that is sensed with the URIR band. As may be noted, a flesh 19 part of the person in the image has a significant contrast relative to the other items in the image 12. The reflectance of the flesh may be only a fraction of the reflectance of the other items, such as the inanimate objects, between 1400 and 2200 nanometers, i.e., the URIR band. This contrast may be seen in image 12 where the flesh 19 of the man 13 appears dark. Everything else in image 12 appears to be light in that each of the reflectance properties appears to be high in the URIR band. In the LRIR band the reflectance properties appear to be normal relative to their color or visual contrast. One may note that a woman 14 in image 11 appears to be rather normal in terms of reflectivity relative to image 11. However, in image 12 she does not exhibit the expected reflectivity of an apparent flesh 21 as the man 13 does. The reason is that she is a mannequin. The other inanimate things such as the radio 15, clothes 16 and hair 17 (including the eyebrows and mustache) may exhibit similar reflectances in images 11 and 12 as the mannequin.

The two images 11 and 12 may have the same field-of-view and be identical to each other on a pixel-to-pixel basis. This basis may make it possible to compare each of the pixels and do a weighted subtraction in terms of brightness, intensity, or contrast of each corresponding pixel pair in the images 11 and 12. The pixel pair signals 22 and 23 from images 11 and 12, respectively, may go to a weighted subtractor 24 for the subtractions. Then the results 25 of the subtractions of the pixel pairs may be thresholded by a thresholder 26 to isolate the pixels of the low reflectance areas, such as the skin or flesh 19, in the URIR band. The remaining pixels may be nullified. The thresholding results 27 from thresholder 26 may be shown by an image 18 which appears to reveal only the skin or flesh 19 pixels of the man 13. The subtraction and thresholding may be accomplished with adder-, subtractor- and/or comparator-like circuitry. The image 18 pixel signals of the flesh 19 of the man 13 may go to a face feature selector and face identifier (selector/identifier) 29. Selector/identifier 29 may seek out the features (e.g., eyes, nose, mouth, ears, head shape, and/or the like) of the face of the man 13 with one or more face feature identification algorithms being applied to flesh pixels 19. The results of the one or more face feature identification algorithms may lead to an identification of the face of the man 13 according to the identified features. Since features of the flesh 19 pixels of hands 31 were not identified by the selector/identifier 29, these flesh 19 pixels may be eliminated thereby leaving only the flesh 19 pixels of the face remaining from the selector/identifier 29. Signal 32 identifying the flesh 19 pixels of the face of man 13 may go to a framer 33. Framer 33 may frame the face flesh 19 pixels with a border that is about 10 to 20 percent or so greater than the image made up by the selected out and identified face flesh 19 pixels of the signals 32. The flesh 19 face pixels with the framing may go as signals 34 to be an image 35 of the framed face 41.

Framed face 41 signals 36 of image 35 may go to a pixel matcher 37 of images 11 and 35 which may superimpose the frame face on image 11 to result in signals 38. Resulting signals 38 from matcher 37 are shown by an image 39. Signals 38 may go to an extractor 42. Extractor 42 may pull out pixels from image 11 that are within the framed face 41 to result in signals 44. Signals 44 may carry a framed face portrait 43 which appears like that of framed face 41 but instead has the pixels of LRIR image 11 rather than those of URIR image 12, and so it appears like a normal-like black and white portrait 43 of man 13. The signals 44 carrying the pixels of portrait 43 may go to a face recognition system 45. System 45 may be a commercial-off-the-shelf system (COTS) from a vendor such as Visionics of Minnetonka, Minn., Cognitech of Pasadena, Calif., or other provider of a face recognition system (FRS). The system 45 may be connected to a database of portraits and/or pictures of various people such as those in a company security database that may be used for identifying employees entering a facility, a criminal database that may be used for identifying suspects, or in any other appropriate database that may be used for various applications.

Signals 47 may provide results of a comparison of portrait 43 with one or more portraits or pictures of person from the database 46. These signals 47 may go to an evaluator 48 which may evaluate the results of the comparison of portrait 43 and provide a confidence level 49 of recognition of the person in the portrait 43.

From a camera 50 in FIG. 2, an input 51 may provide the signal 22 and an input 52 may provide the signal 23. Camera 50 may be focused on a scene 55. Camera 50 may be a several band camera which takes an image 11 in the LRIR band and another image 12 in the URIR band of the scene 55. Images 21 and which are sent as signals 22 and 23, respectively, to a computer 53 of FIG. 2. The images may be processed by the computer according to the operational systems and/or sub-systems shown in FIG. 1. The images may be displayed to a user on a screen 54. The used may interface with the computer 53 via a keyboard 56. Screen 54 and keyboard 56 may be connected to computer 53 via a connection 57. Connection 57 and other connections of the overall system 10 may be hard wired, optical, wireless, or of another manner. Scene 55 may receive sufficient illumination from ordinary daylight. However, under poor light conditions or in total darkness, an illuminator 58 may be used which could emanate infrared light 59 covering the LRIR and URIR bands. The amount of light required may vary depending on other light such as daylight and the conditions of the subject matter such as distance form the camera 50. The sensed images 11 and 12 may provide an indication of the sufficiency the lighting of the scene 55 via signals 22 and/or 23 to computer 53. In response to the sensed lighting conditions, the computer 53 may provide an appropriate intensity control signal 61 to a power supply 62 which may in turn output a certain electrical power via a connection 63 to the illuminator 58 so as to provide proper lighting 59 on the scene 55 under low or no visible lighting.

Various kinds of cameras 50 may be used. One may be a tri-band camera by Honeywell International Inc. of Morristown, N.J. This camera may be capable of separately providing images in the LRIR and URIR bands. Another camera may have two detector arrays of pixels positioned in the camera at about 90 degrees relative to each other with a beam splitter between them at about 45 degrees relative to each array and 45 degrees relative to the lens of the camera facing the scene. The arrays may be co-registered with pixels detecting each portion of the scene 55 the same on a one-to-one pixel basis and in terms of timing for sending the pixel signals to a processor/computer 53. Still another camera 50 may involve two separate cameras, one for detecting LRIR and the other for detecting URIR. The cameras may use filters or other mechanisms for distinguishing and independently detecting the scene in the two different reflective IR bands. These mechanisms may be utilized for a single unit dual band camera. The detecting array may have several sets of pixels on the same array for detecting the scene in two bands. For example, the each pixel of the array may be a double pixel or a set of co-pixels where one may have a filter for LRIR and the other a filter for URIR. There may be another manner for achieving separate two band detection with the pixels on the array. There may be multiplexing techniques that may be used for single camera, single array two-band detection.

In other words, system 10 may capture an image 11 of the scene 55 with a sensing array 111 (FIG. 3) sensitive to the lower reflective IR, and an image 12 of the scene 55 with a sensing array 112 sensitive to the upper band of reflective IR. The images 11 and 12 may have m×n pixels, and have the same field of view of the scene 55.

Each pixel P_((i,j)1) of the image 11 may be located at an i_(th) row and a j_(th) column. Each pixel P_((i,j)2) of the image 12 may be located at an i_(th) row and a j_(th) column. These pixel locations may be instances of the ranges where 1≦i≦m and 1≦j≦m. Each pixel P_((i,j)1) of the image 11 may have a counterpart pixel P_((i,j)2) in the image 12 in a one-to-one mapping of the image 11 to the image 12.

A difference intensity pixel ΔI_(P(i,j)) may be determined from an intensity of each pixel I_(P(i,j)1) of the image 11 and each counterpart pixel I_(P(i,j)) of the image 12, for all of the pixels of the images 11 and 12. Each difference intensity pixel ΔI_(P(i,j)) may have an intensity value that is measured against an intensity threshold value and has its value nullified if it does not meet a certain range of intensity values relative to the intensity threshold value. Each of the ones that do meet it may become a thresholded difference intensity pixel TΔI_(P(i,j)).

Each thresholded difference intensity pixel together with each other thresholded difference intensity pixel may form an image 18 having a one-to-one mapping of pixels to the images 11, 12 and 65. Features of a face may be sought, and if found, a face may be detected in the image 18 and a frame 41 scribed as frame pixels around the face as shown in image 35. Copying the frame 41 pixels from the image 35 to the image 11 may be done on a one-to-one mapping basis and result in image 39. The pixels within the frame 41 in the image 39 may extracted to form an image 43 having the face extracted from the image.

To review, radiation in the middle region of the electromagnetic (EM) spectrum may be regarded as the infrared spectrum. This spectrum includes wavelengths from 0.7 to 100 microns. Within the infrared range, several bands of particular interest may be the 0.7 to 3.0 micron, 3.0 to 5.0 micron and 8.0 to 14 micron bands. The latter two bands are regarded as the thermal infrared band and the first band as the reflected infrared band. The reflected infrared band may be associated with reflected solar radiation that contains no information about the thermal properties of materials. The thermal infrared band, on the other hand, is associated with the thermal properties of materials.

The thermal infrared band may be significant for several reasons. First, the human body maintains a relatively constant temperature of about 37 degrees Celsius (C), irrespective of physical characteristics or illumination conditions. This indicates a consistent light color pattern for the faces of vehicle occupants subject to thermal infrared imaging. This consistency is lacking in the visible spectrum. Such consistency facilitates interpreting sensed images. Further, the thermal property may serve as a differentiator between humans and dummies. Also, a sensor functioning in the thermal region may be operational day and night without any need for an external illumination source.

However, one concern is the attenuation of thermal infrared radiation caused by glass, when detecting humans in a vehicle. The glass may severely disrupt the transmission of infrared radiation at wavelengths greater than 2.8 microns. At 2.8 microns, thermal energy may just begin to appear. To obtain an infrared image under such conditions, one may need a very sensitive mid-infrared camera in the range from 2.0 to 3.0 microns. However, detection with a reflective-infrared camera may significantly reduce this problem of radiation attenuation.

A reflective-infrared camera, if it is restricted to the appropriate range, may output similar imaging signals for various humans despite their having different colors of skin. However, this camera may output a much different imaging signal for a dummy having the same visible color as the human skin.

One system may have a multi-band camera or two cameras of different sensing wavelengths in the reflective-infrared bandwidth. This camera may be pointed toward a place where humans may be detected. An infrared lamp for the illumination of the scene may be used. The outputs of the two cameras or one camera providing the scene in the LRIR and URIR bands may be fused together with a weighted difference to result in an image having an intensified contrast. The image output of the device that performs the fusion of the two camera images may go to a post-processor, which performs binary thresholding on the various pixels of the fused image. The result may be an image with each pixel either being black or white. The thresholded output may undergo such operations utilizing such mechanisms such as fuzzy neural network or analytical processing. The thresholded output may diminish all of the background of the viewed scene except human skin, such as faces.

FIG. 3 shows a basic layout of a reflective-infrared fusion system 110 for detecting humans. It may be a dual-band imaging system. Two co-registered cameras 111 and 112 or one multi-band camera may sense the image of, for instance, a face 113 of a human being. Even though a multi-band camera may be use in place of cameras 111 and 112, for an illustrative example, the discussion here may be in the context of the two cameras 111 and 112. However, the description may be applicable to a single multi-band camera 50 of FIG. 2. Camera 111 may have a spectral sensitivity of 0.8 to 1.4 microns. Camera 112 may have a spectral sensitivity of 1.4 to 2.2 microns. Slightly shorter or longer ranges may also yield acceptable detection results. The 1.4-micron threshold point between the two bands of spectral sensitivity may be a demarcation point for the dual-band system 110, but may be another value as appropriate. Each of the camera sensitivity band ranges may cross somewhat the 1.4 micron wavelength without diminishing the human detecting ability of system 110. The quality of the imaging signals from cameras 111 and 112 may remain high even during overcast days and at nighttime, because of the scene being scanned by cameras 111 and 112, can be illuminated with an eye-safe infrared illuminator 114. Since the eye is not sensitive to the infrared spectrum, system 110 may remain stealthy all of the time, whether in a detection mode or not. The camera at the lower band range (0.8 to 1.4 microns, LRIR) may be an image intensifier. Therefore, the illuminator's spectral emission specification may only need to be in the area of the upper band range (1.4 to 2.2 microns, URIR). The URIR may be quite far from the visible spectrum and illumination in these wavelengths is safe even for highway applications. The infrared cameras 111 and 112 may provide clear imaging signals even in foul weather conditions such as hazy conditions. These particular infrared bands of detection and illumination may provide for sufficient light transmission through windshields, side windows, fog, and darkness. This may permit adequate detection of humans in vehicles at night and in poor weather.

The image outputs 115 and 116 of cameras 111 and 112, respectively, may go to a weighted difference software process 117 or specialized hardware that fuses the outputs by performing weighted subtraction of the intensities of the two camera images. This weighted difference software or hardware may be referred to as a fuser. Such fusion of the camera outputs may intensify the silhouette of face 113 and other exposed human skin in the resultant fused image. Also, the image may feature a diminution of the background of the scene being covered by the cameras. This increased contrast between the person and the background in the fused image may permit excellent image segmentation through thresholding by a software process 118, or specialized hardware. This thresholding software or hardware may be referred to as a thresholder. The output of the thresholder 118 may go to a display 119, printer, or a post-process or specialized hardware.

A final processed image may show the exposed skin parts, such as face 113, as binary blob 113′, as shown in FIG. 3. Background 120 of sensed face 113 may be discounted as shown by blank background 120′ in display 119. This clean-cut binary imagery may ensure reliable and fast operation of a pattern recognition algorithm that identifies a human as indicated by face 113′ imagery.

FIG. 4 illustrates the co-registration of cameras 111 and 112 or image detectors of a multi-band camera. There may be spatial and time registration between the cameras. The cameras may be of the same make and model. A difference between the cameras may be the optical bandwidth filters, 148 and 149, which are situated between sensing arrays 146 and 147 and camera lens 158 and 159, respectively, of cameras 111 and 112. Filter 148 may determine the 0.8 to 1.4 micron spectral sensitivity of array 146 in camera 111 and filter 149 may determine the 1.4 to 2.2 micron spectral sensitivity of array 147 in camera 112. A polarizer may be inserted in front of lens 158 of camera 111 and in front of lens 159 of camera 112. Or instead, a polarizer may be inserted between lens 158 and array 146 of camera 111, and between lens 159 and array 147 of camera 112. Sensing arrays 146 and 147 of the cameras may be the same size, for example, 512 by 512 pixels in a gallium arsenide substrate. Typically, the fields of view are the same for each array. Three pixels 151, 152, and 153, for example, may be selected out as examples of spatial co-registration. Each of the three pixels may be focused on corresponding portions 151′, 152′, and 153′, respectively, of image 120 viewed by cameras 111 and 112. That means the arrays may have the same images, pixel for pixel, even though the spectral sensitivities are different. In other words, the columns and rows of pixels may be aligned with the physical world scene, pixel for pixel. Once spacially co-registered, cameras 111 and 112 may be kept stationary relative to the physical world.

Time co-registration of the cameras may mean that the cameras are in synchronization with each other from a signal perspective. The signals for each of the two corresponding pixels may go to a frame buffer at the same time. The retention of light for each pixel may be in the micro-second range. A typical frame time may be about 33 milliseconds, which is 30 frames per second. The transfer of pixel data may be parallel, line-by-line, or serial, pixel-by-pixel, or any other style of information transfer. There may be a sync signal for cameras 111 and 112 to initiate and maintain their time co-registration.

The image outputs or pixel signals 115 and 116 may go to a software process or specialized hardware 117 which provides a certain weighting to each of the pixels and fuses pairs of corresponding pixels from cameras 111 and 112, respectively, into single pixels. The weighted differencing may be performed pixel by pixel. Each result may be the fused pixel of the two weighted differenced pixels. The weight difference equation for such fusing may be P(i,j)_(fused) =P(i,j)_(lower band) −C*P(i,j)_(upper band).

P is spectral power. The position or location of each pixel in the respective image may be identified by row (i.e., “i”) and column (i.e., “j”). The rows and columns of pixels of the images of cameras 111 and 112 may coincide with each other. The lower band pixels may be from camera 111 and the upper band pixels may be from camera 112. The spectral power “P” for each pixel at i,j may be indicated by a numeral scale of brightness from 0 to 255. “0” may be entirely black or dark (i.e., no spectral power) and “255” may be entirely white or bright (i.e., full spectral power). Numerical indications in between 0 and 255 may be representative of various gradations of gray, brightness, or spectral power. “C” of the equation is a constant (i.e., weighting factor), which may be determined according to the lighting of background or scene 120 and object or face 113. “C” for daytime lighting conditions is about “3” for optimal results. “C” for nighttime depends upon the spectral distribution and properties of the artificial illuminator 114.

The brightness or spectral power of a typical pixel of the lower band (image 146) may be 55 and the brightness or spectral power of the corresponding typical pixel of the upper band (image 147) may be 10. These upper and lower band pixel values may be representative of skin in the corresponding bands. The spectral power of a resulting typical fused pixel, in daytime viewing, at a location of row i and column j in the daytime may be determined with the following calculation. P(i,j)_(fused)=55−3*10 P(i,j)_(fused)=55−30=25

The fused pixel signals may go from software process or specialized hardware 117 to software process or specialized hardware 118 for image segmentation via thresholding of the fused pixels. Process or specialized hardware 118 may emulate a comparator-like circuit in that each fused pixel below a certain threshold value (T) is assigned a value (V₁) of 0 and each fused pixel above the certain threshold value is assigned a value (V₂) of 255.

FIG. 5 shows a histogram of an image frame of fused pixels. The number of pixels for each spectral power value may be indicated by curves 154 and 155 for a given sensed image. The pixels of curve 154 may represent background 120 and the pixels of curve 155 may represent human skin 113. Curves 154 and 155 may intersect at 156 which may be deemed to be the appropriate value for thresholding. If curves 154 and 155 do not intersect, then the thresholding value 156 may be centered between curves 154 and 155. The threshold value is dynamic in that it changes from frame to frame, and may be determined for each image frame according to the histogram of the respective frame. If the spectral value for thresholding is 20, then fused pixels having a value below 20 may be valued at 0 and fused pixels having a value above 20 may be valued at 255. The resulting image in display 119 may have white pixels for background 120′ and black pixels for face 113′. This image may be reversed by process or specialized hardware 118 such that background 120′ has black pixels and face 113′ has white pixels.

A main application of the system may be people detection in places such as parking lots, crowds, stores, airports, military areas, jungles, vehicles, security points, alert areas, and so forth.

The spectrums, where cameras 111 and 112 function, may be within the reflected infrared portion 121 in FIG. 6 which shows the EM spectrum. Visible spectrum 122 may be the spectral sensitivity of conventional cameras. Unfortunately, visible light cameras may have noise levels that increase during poor environmental conditions such as bad weather, nighttime, and direct sunlight. Some problems, such as nighttime viewing may be overcome with artificial lighting, which matches the visible spectrum of the camera, and which in vehicle occupant detection may be a serious distraction to drivers. Another disadvantage is that a human face 113, which may be an object of interest, might not necessarily have consistent qualities within the visible range. Vehicle occupant faces may appear dark or light, depending on the physiological characteristics of the occupant, and the intensity and incident angle of illumination.

The thermal infrared band 123 (3.0 to 5.0 and 8.0 to 14 microns) may be associated with thermal properties of materials. The human body typically may be at a temperature of 37 degrees C. This means that human faces may have a consistent light color in thermal infrared imaging, despite various facial colors, which seems contrary to visible imaging.

The thermal property of the body may provide a clear differentiator from look-alike dummies. The thermal infrared sensor may function at night without an external illuminator. One disadvantage of using the thermal infrared band 23 for occupant detection is that vehicle windshield glass may greatly attenuate infrared light transmission at 2.8 microns and higher.

FIG. 7 reveals the transmittance characteristics of infrared light between 0.4 and 2.8 microns for a clean vehicle windshield (curve 124) and a dirty windshield (curve 125). Beyond 2.8 microns of thermal infrared bandwidth 123, the radiation transmittance characteristic of windshield glass may be almost zero. The transmittance of a lightly tinted side window of a vehicle may be good (50 to 85 percent), from 0.3 up to 2.8 microns as revealed by curve 126 in FIG. 8. Between 2.8 and 4.3 microns, the radiation transmittance is about 20 percent for the side vehicle window. Beyond 4.3 microns the transmittance drops to nearly zero. However, the spectral behavior of the side window may permit transmittance of some thermal radiation.

Curves 127 and 128 of FIG. 9 show the percentage of reflectance of infrared light for light and dark complexion Caucasian males, respectively. The reflectance is good between 0.6 and 1.4 microns. Above 1.4 microns, the reflectance is significantly diminished. However, the difference of reflectances of light and dark complexions is minimal.

In FIG. 10, curves 129 and 130 show skin reflectances for light and dark complexions, respectively, of Asian males. Curves 131 and 132 of FIG. 11 show skin reflectances for light and dark complexions of black males. Even though the reflectances of light complexions may be higher than those of dark complexions; curves 127, 128, 129, 130, 131 and 132 of FIGS. 9-11, respectively, have similar shapes and all of them appear to drop off at about 1.4 microns. These reflectances may reveal some variation for all complexions of the Caucasian, Asian, and black males, between 0.6 and 1.4 microns.

Curves 133 and 134 of FIG. 12 may show the reflectances for more extreme differences of light skin and dark skin, respectively. The reflectances of light and dark skin may be significant up to 1.4 microns. After 1.4 microns, reflectance curves 133 and 134 for light and dark skin, respectively, may become almost coincident and the resultant reflectances may drop below 20 percent. Thus, in the reflective-infrared bands of 1.4 microns and above, the reflectances of detected humans of all kinds of skin shade may be about the same at wavelengths greater than 1.4 microns.

In FIG. 13, curves 135, 136, and 137 show the reflectance for cotton, wood, and polyamide respectively, to be significant not just between 0.6 and 1.4 microns, but beyond 1.4 microns. The insignificant drop in reflectance after the 1.4 micron threshold point may be a basis that shows a large contrast in reflectance between the human face and inanimate objects, such as upholstery, the dashboard, and fabrics in a vehicle, which are background, and may provide for easy detection of the human face in the range of 1.4 to 2.2 microns.

Consequently, there may be a significant contrast, in reflectances between the images of a Caucasian male and a dummy head from a camera operating in the range between 1.4 and 2.2 microns. The image of the dummy may be reflective and appear rather bright and the male human image may be non-reflective and appear dark, and thus the resultant contrast between the two images appears to be rather stark. The person detection scheme seems superior to visible light imaging, since the latter scheme reveals little or no contrast between the Caucasian and dummy heads of like colors. Thus, it may be easy to distinguish images of the human head from those of the dummy head in the 1.4 to 2.2 micron range imagery, but not be necessarily easy to distinguish the images of those respective heads from each other in the 0.8 to 1.4 micron range imagery.

The lower reflectivity of human skin for the 1.4 to 2.2 micron spectral range may be explained by the spectral reflectance of distilled water as shown by curve 138 of FIG. 14. There is a substantial drop in reflectance at about 1.4 microns. Beyond 1.4 microns, the water absorbs substantial infrared radiation and appears in an image as a dark body. Since the composition of the human body consists of 70 percent water naturally, its spectral response may be similar to that of water. So camera 112, operating in the 1.4 to 2.2 micron range, may capture this unique human body differentiator. With the operating range of camera 112, one may safely use during the nighttime the matching reflective-infrared illumination source 114 to improve the quality of the sensed image of face 113. The light of source 114 may be invisible to humans, but also harmless to their eyes since the wavelength of illuminator or source 114 may be in a safe range.

Also, since cameras 111 and 112 of system 110 may operate at a lower band than the mid-infrared band, glass penetration should not be a problem and cameras 111 and 112 may easily detect through the frontal windshield of a vehicle, a building window such a door, a glass observation panel, or the like. Thus, speed requirements for cameras 111 and 112 may be less restrictive. For an instance at a highway site, a zoom lens may be used with the camera arrangement for detection and observation of people.

FIG. 15 shows an illustrative example involving a layout of a reflective-infrared system 140 for determining the speed characteristics of the human detector. A vehicle 141 may be assumed to be moving down a highway at a velocity v, as shown by vector 142, and be observed in a frontal view with a reflective-infrared camera 111 or 112 at a distance d, as shown by line 143, and from a height h, as shown by line 144. Only one of the cameras 111 and 112 may be needed for this evaluation, but it may be either one of them. Cameras 111 and 112 may be a Sensors Unlimited Inc. SU 320 equipped with a telephoto lens, a band-pass filter in the range of about 0.8 to x microns (where x>0.8 microns), and a polarizing filter to reduce the glare effect from the sun illumination during daytime.

During the daytime, system 140 may have adequate illumination from the sun. There may be an appropriate geometric arrangement for cameras 111 and 112 so that the signal to noise (S/N) ratio and the camera speed can be kept at acceptable levels even under adverse conditions. An acceptable (S/N) ratio may be above 35. The speed quality may be considered acceptable when the image smearing does not exceed the width of one pixel.

The first step in a radiometric computation may be to determine the amount of radiation that falls upon the objects of interest such as the occupants of vehicle 141. The spectral band considered may be above the 1.4-micron threshold point. Because of constraints due to the quantum efficiency of the camera SU-320 that may be used in the actual experiments, one may limit the spectral band in the range of 1.4 to 1.7 microns. Slightly modified things may be in effect for the extended range of 1.4 to 2.2 microns. The spectral irradiance of the sun (the illumination source) on a clear day at sea level may be approximately I_(sunny)=0.008 Watts/cm² in the 1.4 to 1.7 micron band range. In this computation, however, one may consider the worst case scenario of an overcast day. For an overcast day, the irradiance value may be reduced by 10⁻³ thus giving irradiance at vehicle 141 of approximately $\begin{matrix} {I_{overcast} = {10^{- 3}*I_{sunny}}} \\ {= {10^{- 3}*0.008}} \\ {= {8\quad{microwatts}\text{/}{{cm}^{2}.}}} \end{matrix}$

The transmittance in this spectral range of windshield 145 of vehicle 141 may be approximately 0.4 resulting in an irradiance on the vehicle occupants of $\begin{matrix} {I_{occupant} = {0.4*I_{overcast}}} \\ {= {0.4*8}} \\ {= {3.2\quad{microwatts}\text{/}{{cm}^{2}.}}} \end{matrix}$

The second step in a radiometric computation may be to determine how much of the incident radiation on the objects of interest is reflected back to the sensor (i.e., reflective-infrared camera 111, 112). The radiance into a hemisphere assuming a reradiate of 0.4 may be $\begin{matrix} {R_{occupant} = {0.4*{I_{occupant}/\pi}}} \\ {= {0.4*{3.2/\pi}}} \\ {= {{0.4\quad{microwatt}\text{/}{cm}^{2}} - {steradian}}} \end{matrix}$

This may represent the reflected portion of the occupant irradiation. The occupant's body may absorb the rest. The reflected radiation may pass through windshield 145 and the camera 111, 112 lens to reach the reflective-infrared sensor array of camera 111, 112. One may assume a 0.4 windshield transmittance, an f/2 camera lens (i.e., having a 14.32 degree cone angle) with 0.8 transmittance, a polarizer with 0.4 transmittance, and a band-pass filter with 0.6 transmittance. Then, the irradiance at the sensor array of camera 111, 112 may be $\begin{matrix} {I_{camera} = {0.4*0.8*0.4*0.6*\pi*R_{occupant}*{\sin^{2}\left( {14.32{^\circ}} \right)}}} \\ {= {0.4*0.8*0.4*0.6*\pi*0.4*{\sin^{2}\left( {14.32{^\circ}} \right)}}} \\ {= {0.006\quad{microwatt}\text{/}{{cm}^{2}.}}} \end{matrix}$

Camera 111, 112 may have square pixels with a side of 37.5*10⁻⁴ cm or an area $\begin{matrix} {A = {37.5*10^{- 4}*37.5*10^{- 4}}} \\ {= {1.40*10^{- 5}\quad{{cm}^{2}.}}} \end{matrix}$

Consequently, the radiant power on the camera 111, 112 pixel may be $\begin{matrix} {P_{pixel} = {A*I_{camera}}} \\ {= {1.4*10^{- 5}*0.006}} \\ {= {0.084*10^{- 12}\quad{{watts}.}}} \end{matrix}$

The camera's detectivity D* may be D*=10¹² cm Hz/Watts. The noise equivalent power (NEP) may be related to detectivity D*, pixel area A, and electronic bandwidth Δf by the following equation NEP=(A/Δf)^(1/2) /D*.

The bandwidth Δf may be determined by the exposure time of camera 111, 112. The exposure time may depend on vehicle 141 velocity 142, camera range 140, and the camera 111, 112 field of view such that the images smear less than one pixel. Assuming vehicle 141 traveling at a speed of 65 mph, at a distance d 143 of 40 meters (m) away from camera 111, 112, and with a field of view of 1.6 m, the 320×240 pixel array of camera 111, 112 may give a maximum exposure time of 1 ms or a bandwidth of Δf=1 kHz. Substituting the values for A, Δf, and D* in the formula of NEP, one may get NEP=1.18*10⁻³ watts.

Therefore, the signal to noise ratio S/N may be S/N=(P _(pixel) /NEP)=0.7.

Assuming a worst case scenario (overcast day, dirty windshield, dark occupant skin), one may determine that camera 111, 112, equipped with an f/2 lens, a 1.4 to 1.7 micron filter, and a polarizer, if it is positioned at a distance 143 of d=40 m from incoming car 141 and at a height 144 of h=7 m at the specified distance 143, will achieve an acceptable smear of less than one pixel since the required exposure time of 1 ms is within the camera's speed capabilities. The signal to noise ratio (S/N) may be 0.7. To boost the S/N ratio to a higher value on overcast days, one may employ an illumination source 114. Illumination source 114 may also be helpful during nighttime. If one operated in the visible spectrum, the use of an illuminator on occupied vehicles may be prohibitive. However, the spectral signature of illuminator 114 for the 1.4 to 1.7 micron wave-band might be safely employed on occupants and drivers of moving vehicles.

Post processing may include a neural network that performs automatic vehicle occupant detection. The vehicle occupant detection approach may be based upon a fuzzy neural network algorithm. An exact binary image may be provided by the fusion approach described and facilitate high correct detection rates.

The face detection apparatus and system described herein may be used as a preprocessing phase for a face recognition technique as shown in FIG. 16. FIG. 16 shows a person identification system 210 which may be operable under control of a user at an interface of a processor 216 to implement face detection algorithms 220 in a pre-processing stage prior to application of face recognition algorithms 222.

As shown in FIG. 17, a person identification system 250 that may be implemented with the system 210 of FIG. 16 may include detecting a face which exists in a scene being monitored (block 252), e.g., determining the location of eyes of the face for use in defining the orientation and/or extent of the face. With such information available from the face detection process, other face recognition algorithms 222 may be used (block 256).

Face detection may be effective when the head of person is at various positions relative to the imaging devices acquiring data with regard thereto. For example, as shown in FIG. 18, the head of a subject facing forward as well as most degrees of rotation may be considered. Such degrees of rotation shown by the head in FIG. 18 may include head orientations within a −35 degree to +35 degree range around the z-axis, within a −35 degree to +35 degree range around the x-axis, and within a −90 degree to +90 degree range around the y-axis.

The face detection process 252, e.g., sub-system, of the person identification system 250 may include imaging hardware 212 and a computer apparatus 216 operable to apply face detection algorithms 220 to image data provided by the imaging hardware 212. After application of the face detection algorithms 220 and upon detection of a face of a person, image data representative of at least a portion of the person's face may be provided to the face recognition system, e.g., visible image data, reflective-infrared data, and the like, with information determined as part of the face detection process. For example, data representative of the center of eyes of the face detected, and/or one or more other facial features, may be provided to the face recognition system to allow effective analysis of the image data representative of the currently detected face in the scene being monitored.

The face recognition algorithms 222 may be any algorithms used for identification of an individual or analysis of an individual's face, e.g., reflective-infrared analysis. Such analysis may be used to determine the anxiety of a person. In the illustrative example shown in FIG. 16, the face recognition algorithms 22 may include a face data unit 232 for storing data representative of individuals, e.g., facial features, facial images, facial signatures, and the like.

The face recognition algorithms 222 may further include face recognition analysis tools 234 for analyzing image data concerning the face detected by one of more of the face detection techniques described herein. For example, the analysis tools 234 may be operable to compare stored data of face data unit 232 to features of image data provided regarding the detected face, e.g., visible image data, reflective-infrared image data, or any other data that may be beneficial in the identification of an individual or even be beneficial in the assessment of an individual's intent, e.g., terrorist, smuggling, and so forth.

One or more of the face detection techniques described herein may be used to modify or be incorporated into currently available face recognition systems. In one illustrative, one or more of the techniques may be incorporated into a system that includes the face recognition engine FaceIt.RTM available from Visionics (Jersey City, N.J.). Since FaceIt.RTM may rely primarily on facial geometry for face recognition, it might be invariably applied to visible as well as reflective-infrared imagery. In other words, the nominal face detector in the FaceIt.RTM system may be replaced with the face detector.

Various imaging hardware system configurations 212 may be used in the implementation of the face detection process to provide reflective-infrared image pixel data for a scene (block 272 as shown in FIG. 20). However, as shown in FIG. 19, the imaging hardware configuration 212 may include a multiple reflective-infrared band camera approach. The system may use a camera 260 (i.e., having sensing arrays 146 and 147 of cameras 111 and 112) as the input medium. The camera 260 may be sensitive to the so called reflective-infrared spectrum in the range of about 0.9 microns to about 1.7 microns. This range falls within the reflected portion of the infrared spectrum and has no association with thermal emissions.

In one illustrative example, as may be further described herein, the two sensing arrays 146 and 147 of camera 260 may be set at perpendicular angles with a beam splitter used to allow both arrays of camera 260 to view the scene from the same vantage point, yet in different wavelength bands. The splitter may divide the light reflected from the scene into a lower band beam of about 0.9 microns to about 1.4 microns in the reflective-infrared spectrum and the upper band beam of about 1.4 microns to about 1.7 microns. The two beams may be funneled to the sensing focal plane arrays (FPA) of the camera 260. Each array of camera 260 may be connected to a frame grabber 262, which digitizes the incoming image data, e.g., video, for sending on to computer 216.

A luminance monitor component 230 of the software 220 running on the computer apparatus 216 with the frame grabbers 262, may analyze the luminance in the incoming frames of image data. The system may then control an illumination source 268, e.g., a reflective-infrared lamp to provide the desired illumination of the scene. For example, the system may appropriately adjust the output voltage on a programmable power supply unit 266 connected to the computer 216 via the serial port. The power supply 266 may provide power for the reflective-infrared lamp 268 that illuminates the scene. Through this feedback, the system may keep the scene at a constant luminance regardless of external conditions.

In other words, a computer controlled reflective-infrared illumination source may be added to the system to maintain optimal illumination levels in the scene at all times. For example, a photometer may be used to sense scene illumination and provide a signal to initiate the need for computer adjustment of the illumination source. As opposed to constant illumination of the scene, the lamp 268 may be strobed when a subject gazes at the system unknowingly for too long.

Generally, the face detection algorithms 220 operable on the reflective-infrared image data provided by the imaging device hardware system 212, such as described above, may include four units or modules as shown in FIG. 16. The face detection algorithms 220 are operable for use in carrying out the face detection method 252 shown in FIG. 20.

The face detection system 252 may include receiving reflective-infrared image pixel data for a scene, or in other words, recognizing the reflective-infrared image data provided by the imaging device hardware system 212 (block 272). Skin in the scene may be isolated versus background based on the reflective infrared pixel data and be indicative of a face in the scene (block 274). Thereafter, if skin is isolated in the scene, a facial search area or space may be defined that limits the region of the scene in which a search for eyes of a person can be conducted (block 276). For example, the facial search space may be refined by various imaging techniques to present a search space representative of a silhouette of the detected person's exposed facial skin. After the facial search space has been defined, an eye location process may be applied to locate the eyes in the defined facial search area (block 278).

The input process unit 224 may acquire the input images for both upper and lower reflective-infrared bands from the frame grabbers 262 (block 272). The images may then be aligned and sent to the luminance monitor 230 and the skin detector module 226.

The luminance monitor 230, as described above, may evaluate the current luminance level in the scene and dynamically adjust the power output on the power supply 266. A simple mapping between the output voltage and the corresponding luminance, as shown in FIG. 21, may allow the system to accurately achieve the desired light level in the scene.

Upon receiving the dual input images, generally, the skin detector module 226 may perform a series of operations to isolate the skin in the image data (block 274 and block 276). For example, the output of the skin detection module 226 may be a binary image where all skin appears black against a white background. This image may then be passed to the final unit of the face detection software 220, the face detector module 228.

Generally, the face detector module 228 may locate one or both eyes of the face such that, for example, orientation of the face may be determined. For example, a template process may be used on a composite feature image extracted from the dual reflective-infrared images and the skin image to provide a good approximation of the location of the eyes. Based on the location of the eyes and/or skin, the orientation and extent of the face may be determined heuristically. Such determination may be performed employing certain known orientation characteristics.

For example, a normal line may be defined extending in the y-axis direction (see FIG. 18) orthogonal to the z-axis which splits the eyes of the face being detected. If the person is facing directly forward, generally the normal line may be equidistant between the eyes detected. However, if the person is turned, e.g., head rotated slightly around the y-axis, then one eye will be closer to the normal line than the other.

Similarly, for example, a normal line may be defined extending in the x-axis direction (see FIG. 18) orthogonal to the y-axis which splits the face into an upper and lower region along the level eyes on the face. The eyes may generally be located at a certain position on the face, e.g., with ⅓ of the face above the eyes and ⅔ of the face below the eyes, or in other words below the normal line. If the person is facing directly forward, generally (at least in the example line provided above) ⅓ of the skin detected may be above the normal line and ⅔ below the normal line. However, if the person's head is leaning forward or backward, e.g., head rotated slightly around the x-axis, then a different ratio of skin above the normal line and skin below the normal line may be apparent. For example, such analysis may be used to eliminate portions of detected skin associated with the neck.

Computer apparatus 216 may include a processor operable to execute software to provide a user with operations noted herein, e.g., face detection. As such, computer apparatus as used herein may include not only circuitry such as processors capable of executing various kinds of software, but may also include specialized hardware.

For example, the computer system 216 may be any fixed or mobile computer system, e.g., a personal computer. The exact configuration of the computer system is not necessarily limiting and most any device capable of providing suitable computing capabilities may be used. Further, various peripheral devices, such as a computer display, a mouse, a keyboard, memory, printer, and so forth, may be contemplated for use in combination with a processing apparatus of the system.

With further reference to the receipt of reflective-infrared image data (block 272) for the face detection method 252 of FIG. 17, various examples of providing such reflective-infrared image data and the information that may be obtained using such data may be described herein with reference to FIGS. 3-15. The reflective-infrared spectrum may be particularly beneficial for skin detection purposes.

Human skin may exhibit an abrupt change in reflectance around 1.4 microns. This phenomenology allows for highly accurate skin mapping by taking a weighted difference of a lower band reflective-infrared image and an upper band reflective-infrared image. One advantageous consequence of the phenomenological basis of the skin detection or isolation method may be that artificial human heads or disguised heads do not fool the system.

The skin detector module 226 may use at least a portion of the lower and upper band of the reflective-infrared spectrum to isolate skin versus background in the scene being monitored as reflected generally in block 274 of the face detection method 252 of FIG. 20. As used herein, the portion of the upper band of the reflective-infrared spectrum used may include a portion in the range from 1.4 microns to at least about 1.7 microns. As previously indicated herein, at 2.8 microns thermal energy may begin to appear. As used herein, the portion of the lower band of the reflective-infrared spectrum may include a portion in the range from at least about 0.9 to 1.4 microns.

One illustrative example of the skin isolation process (block 274) of the face detection method 252 shown generally in FIG. 20 may be performed using the concepts described herein. The pixel mapping (i.e., pixel pair) for the fusion of the two reflective-infrared images (i.e., the upper band and lower band images) is as described herein and may be as follows: P(i,j)_(fused) =P(i,j)_(lower) −f*P(i,j)_(upper) where, P(i,j)x is the pixel value at position (i,j) in the respective image x and f may be the weight factor used (previously listed as constant C). For example, through experimentation it may be determined that a near optimal value for f is 1.38, which may be different for daytime lighting conditions. The weighted subtraction operation may increase substantially the contrast between human skin and the background in the image data.

Following the weighted subtraction process, a thresholding operation may be applied. Any suitable thresholding process that provides for acceptable segmentation of dark and light regions may be used. In general, such thresholding processes compare the data representative of the reflection to one or more thresholding values. Such values may be based on a variety of factors, such as the reflection characteristics of natural skin, of natural hair, background, and so forth. For example, a thresholding process described in the article entitled “A Threshold Selection Method from Gray-Level Histograms” by Otsu, IEEE Transactions on Systems, Man and Cybernetics, Vol. SMC-9, No. 1, January 1979, may be used. The thresholding process may generally involve a non-parametric and unsupervised method of threshold selection. An optimal threshold may be selected so as to maximize the separability of the resultant classes in gray levels. The algorithm utilizes only the zeroth-order and the first-order cumulative moments of the gray level histogram. The speed of the system, in part because of the thresholding process, may provide real time images to the user.

An illustrative skin isolation process is shown in the FIGS. 23 a-23 d. FIG. 23 a illustrates a lower reflective-infrared band image wherein the skin has a high reflectivity and FIG. 23 b illustrates an upper reflective-infrared band image wherein the skin has a relatively low reflectivity when compared to the high skin reflectivity in the upper lower band. The weighted subtraction image is shown in FIG. 23 c and provides a sharper contrast between skin and background. Further, following application of a thresholding algorithm, the binary image of FIG. 23 d may be attained.

After the thresholded image is attained, which is representative of a facial search area which can be searched, for eyes of the person, further definition may be provided with respect to the binary image to reduce the search area. This may result in saved processing time later when other algorithms are applied to pixels in the facial search area.

For example, as shown in FIGS. 23 e-23 h and described with reference to FIG. 22, such further definition processing may include providing the image pixel data for skin isolated with respect to background, e.g., the binary image (block 280) as shown in FIG. 23 d. Thereafter, opening and closing operations may be performed on the image data to smooth the representation of the skin or outline thereof (block 282). In other words, the binary image may undergo a series of morphological operations.

For example, the opening operation may smooth the contour of the skin region, breaks narrow isthmuses, and eliminate small islands and sharp peaks or capes. An open image is shown in FIG. 22 e. The closing operation may fuse narrow breaks and long, thin gulfs; eliminates small holes; and fills gaps on the contours. A closed image is shown in FIG. 23 f. Such opening and closing techniques may be achieved with techniques of image processing.

Further, the application of dilation and erosion algorithms may be used to result in the elimination of small image detail (block 284). A dilated image is shown in FIG. 23 g and an eroded image is shown in FIG. 23 h.

Although several various image processing techniques have been described, there may be other techniques that may be used to further define the facial search area. This defined search area may provide data for determining the extent of the detected face.

With the facial search area defined as the isolation of skin in the scene, the eye location process 278 of the face detector method 252 may be applied to pixels that fall in the defined area. However, it should be recognized that the eye location process may be applied to a much larger area without having defined a facial search area to locate eyes of a person. However, the skin may be isolated relative to background to provide a smaller region upon which to apply the eye location process 278.

A goal of the face detection method 252, and particularly the eye locator process 278, may be to determine the orientation and extent of the face 213 of a detected person. This goal may be attained by finding the location of one or both eyes of the face, in the defined facial search area, as may be described with reference to FIGS. 24-29.

The face detection method 252 may accurately determine the location of at least one eye in order to provide information of some use to a face recognition system. A major strength may be the exploitation of the phenomenology exhibited by the skin, eyes, and hair in the reflective-infrared band of the EM spectrum.

The face detection method 252, and particularly the eye locator process 278, may use a three-step approach to determine the location of one or both eyes as shown in FIG. 24. Upon receipt of image data for the defined facial search area (block 290), e.g., the high and low band reflective-infrared images as shown in FIGS. 23 a and 23 b, respectively, and the output image from the skin detector module as exemplified in FIG. 23 h, an extract process 291 may be performed. The extraction process 291 may extract regions in the upper and lower reflective-infrared images that are likely to be the eyebrows (see, for example, FIG. 27 a) and eyes (see, for example, FIG. 27 b). The extracted regions likely to be the eyebrow and eye features may then be fused into a composite feature image (see, for example, FIG. 27 c).

Thereafter, the composite feature image may be searched based on model data representative of an eye region, e.g., standardized data generally representative of a plurality of persons facial characteristics (block 298), to located eye containing regions. The center of one or both eyes of the face may then be estimated within the located eye containing regions (block 600).

There may be an illustrative face detection method 252, and particularly eye locator process 278. First, the eye locator process 278 may include extracting regions in the upper and lower reflective-infrared images that are likely to be the eyebrows (block 292) and likely to be eyes (block 294), respectively. This extraction may be accomplished by capitalizing upon the unique reflectance characteristics of human hair and skin in the upper and lower reflective-infrared bands.

In the upper reflective-infrared band, eyebrow hair may stand out comparatively to the extremely low reflectivity of human skin. The threshold values found to be suitable for the eyebrow extraction may be as follows: B(P _(u)(x,y))={0.65<P _(u)(x,y)<80 {255,65>=P _(u)(x,y) or P _(u)(x,y)>=80 where B(·) is the eyebrow threshold function, and P_(u)(x, y) is the pixel value of the upper reflective-infrared image at position (x, y).

In the lower reflective-infrared band, the eyes may stand out comparatively to the extremely high reflectivity of human skin. The threshold values found to be most suitable for the eye extraction may be as follows: E(P ₁(x,y))={150,30<P ₁(x,y)<90 {255,30>=P ₁(x,y) or P ₁(x,y)>=90 where E(·) is the eye threshold function, and P₁(x, y) is the pixel value of the lower reflective-infrared image at position (x, y).

FIG. 27 a shows at least one eyebrow region 500 extracted with other regions that meet the threshold. Further, FIG. 27 b shows at least one eye 502 extracted with other regions that satisfy the threshold.

The extracted eyebrow and eye feature images may then be fused into a composite feature image (block 296). As shown in FIG. 27 c, a tri-level image may be presented wherein the black areas denote likely eyebrow regions, gray areas denote likely eye regions, and the white areas denote all the rest.

As may be seen therein, the actual eyebrow region 500 and eye region 502 are in close proximity to one another. This relationship may be used to apply a template process to search the composite image to locate eye containing regions based on model eye region data (block 298).

One illustrative example of the searching method 298 may be described with reference to FIG. 25. As shown in FIG. 25, the searching method 298 may include receiving image data representative of the composite image including the extracted regions (block 300). A template model representative of the position of eyes and eyebrows, e.g., a template normalized over a plurality of persons, may be applied to the composite feature image including the extracted regions (block 302). For example, the template may model the appearance of an eye region in the composite feature image, given the constraints of human anatomy. Thereafter, image data representative of any found or resultant eye containing regions may be provided for further analysis (block 304).

In one illustrative searching method 298, a template process, e.g., a Hough transform, may be used to find the eye containing regions of the composite feature image. For example, a generalized Hough transform template, such as described by D. H. Ballard, “Generalizing the Hough Transform to Detect Arbitrary Shapes,” Pattern Recognition, Vol. 13, No. 2, pp. 111-122 (1981), may be modeled after the expected appearance of an eye region in the composite feature image. An example template 508 is shown in FIG. 28. The template 508 may include a black region 510 (modeling the eyebrow) over a gray region 520 (modeling the eye). The template 508 may be rotated and sized at each point of implementation to account for the rotation and variation of individual faces. The result of this transformation may be the location of one or more eye containing regions.

For example, FIG. 27 d is an example representation of the result of applying the template 508. The result may be a tri-level image. The background 530 shows as white, the skin region as gray 532, and within the skin region the area(s) that exhibited the strongest response to the eye template are shown as black (e.g., eye containing region 534). In other words, FIG. 27 d shows the result of the Hough Transform superimposed on the defined skin image. As can be seen in FIG. 27 d, the border 536 is representative of the defined search area that outlines the detected or isolated skin. The extent of the face may be dependant on parameters and/or characteristics of such a border.

One illustrative example of the eye center estimation process 600 of the eye locator method 278 is shown in FIGS. 29 a-29 c, and may be described with reference to FIG. 26. The estimation process 600 may receive the data representative of the eye containing regions (block 310) in association with the defined isolated skin region (see, for example, FIG. 27 d). The process 600 may determine what type(s) of eye containing region(s) have been located (block 312). After the type(s) of eye containing region(s) have been identified, then the approximate center of one or both eyes may be estimated (block 314).

For example, the center of the subject's eyes may be estimated through blob analysis. A blob may refer to a group of connected pixels. Because of the variation in human faces, many different patterns of “eye” blobs, which correspond to eye containing regions, may arise in the resulting Hough Transform image in three cases as shown in FIGS. 29 a-29 c.

In the case of FIG. 29 a, there is a single blob that spans the width of the face region. This blob may be bisected in the middle and processed as two smaller blobs to determine the center of both eyes contained therein. In the case of FIG. 29 b, there are two blobs that are roughly equal size and which are generally of a larger size than any other blobs. The centers of both eyes represented thereby may be determined. In the case of FIG. 29 c, there may be a single small blob set apart from and of a larger size than any other blobs. Here, only a single eye center is determined.

Ultimately, the face detector may locate the center of one or both of the eyes. For example, the center may be taken as the centroids of the selected blobs. However, as the blobs represent not only eyes, but also eyebrows, the center of the eyes will not be the centroid but may generally be found as a function of the centroid of such blobs, e.g., offset from the centroid by a particular distance.

Once the center of the eyes is located, then the orientation of the head may also be determined. For example, if the eyes fall within a certain region of the isolated skin, then it may be determined that the head is at a certain position, e.g., the subject may be looking down or up.

After the orientation and location of the head are known then it may be possible to ascertain if a good image of the face can be extracted for recognition purposes. It may also be a viable option to extrapolate where necessary to create a frontal image of the face from any given rotation, provided of course that at least half of the face is visible.

In the present specification, some of the material may be of a hypothetical or prophetic nature although stated in another manner or tense.

Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. 

1. A detection system comprising: a first array of detector pixels having a first field of view; a second array of detector pixels having the first field of view; a subtractor connectable to each pixel pair of the first and second arrays; and a comparator connected to the subtractor; and wherein: the first sensing array has a sensitivity to light in a lower portion of a reflective infrared spectrum; the second sensing array has a sensitivity to light in an upper portion of a reflective infrared spectrum; a pixel pair includes a first pixel at a location of the first array of detector pixels of the first array which a one-to-one correlation with a second pixel of the pixel pair at a corresponding location of the second array of detector pixels; the subtractor may detect the differences of magnitudes of output signals from the first and second pixels of each pixel pair and output a magnitude difference signal for each pixel pair; and a comparator may compare the magnitude difference signals to a certain threshold value and select the pixel pairs having magnitude difference values on one side of the threshold value.
 2. The system of claim 1, wherein: the second pixels of each of the pixel pairs having a magnitude difference value on one side of the threshold value exhibit detected low reflectance areas in the field of view; and the low reflectance areas may indicate skin of a person.
 3. The system of claim 2, further comprising: a face feature selector; a framer; and an extractor; and wherein: an image from of the second array of detector pixels may reveal the skin of a person in the field of view; the feature selector may locate features of a face in the shin; the framer may place a frame around the face in the image; pixels of the frame around the face may be matched with pixels at corresponding locations in an image from the first array of detector pixels; and the extractor may extract a sub-image containing the face within the frame in the image from the first array of detector pixels.
 4. The system of claim 3, further comprising: a face recognition system; and wherein: the face recognition system may match the sub-image of the face with another sub-image of an identified face.
 5. The system of claim 4, further comprising: an evaluator; and wherein the evaluator may indicate a confidence level of recognition of the face within the frame.
 6. The system of claim 5, further comprising a database of sub-images of identified faces, connected to the face recognition system.
 7. A human detection system comprising: a multi-band camera having first and second outputs; a subtractor connected to the first and second outputs; a thresholder connected to the subtractor; a feature selector connected to the thresholder; a framer connected to the thresholder; a pixel integrator connected to the framer and the first output; an extractor connected to the integrator; a recognition system connected to the extractor; and
 8. The system of claim 7, further comprising an evaluator connected to the recognition system.
 9. The system of claim 8, wherein: the first output of the multi-band camera has a bandwidth in the lower reflective region of the infrared spectrum; and the second output of the multi-band camera has a bandwidth in the upper reflective region of the infrared spectrum.
 10. The system of claim 9, wherein the subtractor may remove pixels from an image which are not representative of skin.
 11. The system of claim 10, wherein: the feature selector seeks out features of a face in the image; the feature selector may retain only the face in the image; and the framer may output a framed image of the face.
 12. The system of claim 11, wherein: the pixel integrator may match the pixels of the framed image with an image from the first output of the multi-band camera; and the integrator may superimpose the frame on the image from the first output of the multi-band camera.
 13. The system of claim 12, wherein the extractor may extract a portion of the image within the frame.
 14. The system of claim 13, wherein: the portion of the image may be sent to a recognition system; and the recognition system may perform a match of the portion of the image with a similar image from a data system.
 15. The system of claim 14, wherein the evaluator may provide a confidence level of recognition of the match.
 16. The system of claim 15, further comprising a light source for providing infrared lighting as needed.
 17. A method for detecting a face in a scene, comprising: capturing a first image the scene with a first sensing array sensitive to a first band of reflective infrared light; capturing a second image of the scene with a second sensing array sensitive to a second band of reflective infrared light; and wherein: the first image comprises m×n pixels; the second image comprises m×n pixels; the first and second images have the same field of view of the scene; each pixel P_((i,j)1) of the first image is located at an i_(th) row and a j_(th) column; each pixel P_((i,j)2) of the second image is located at an i_(th) row and a j_(th) column; 1≦i≦m; 1≦j≦m; each pixel P_((i,j)1) of the first image has a counterpart pixel P_((i,j)2) in the second image in a one-to-one mapping of the first image to the second image; a difference intensity pixel ΔI_(P(i,j)) may be determined from an intensity of each pixel I_(P(i,j)1) of the first image and each counterpart pixel I_(P(i,j)) of the second image, for all of the pixels of the first and second images; each difference intensity pixel ΔI_(P(i,j)) has an intensity value that may be measured against an intensity threshold value and has its value nullified if it does not meet a certain range of intensity values relative to the intensity threshold value, and becomes a thresholded difference intensity pixel TΔI_(P(i,j)); each thresholded difference intensity pixel together with each other thresholded difference intensity pixel forms a third image having a one-to-one mapping of pixels to the first and second images; and features of a face are sought, and if found, a face is detected in the third image and a frame is scribed as frame pixels in the third image around the face.
 18. The method of claim 17, further comprising: transferring the frame pixels from the third image to the first image on a one-to-one mapping basis; and extracting the pixels within the frame in the first image to form a fourth image having the face extracted from the first image.
 19. The method of claim 18, further comprising: comparing the face in the fourth image with faces from a database; and matching the face in the fourth image with a face from the data base.
 20. The method of claim 19, further comprising: evaluating the matching of the face in the fourth image with the face from the database; and determining a confidence level of recognition of the face in the fourth image relative to the face from the database.
 21. The method of claim 20, wherein: the first band of reflective infrared light is approximately between 800 and 1400 nanometers; and the second band of reflective infrared light is approximately between 1400 and 2200 nanometers.
 22. The method of claim 21, further comprising shining infrared light on the scene as needed for obtaining adequate first and second images of the scene.
 23. Means for detecting faces, comprising: means for obtaining a first image with a first field of view of a scene at a lower reflective infrared band of light; means for obtaining a second image with the first field of view of the scene at an upper reflective infrared band of light; means for subtractively combining the first and second images into a third image; means for thresholding the pixels of the third image to result in a fourth image having only the pixels meeting a set threshold value; means for finding features of a face in the fourth image; means for inserting a frame around the face based on the features; and means for transferring the frame to be around the face in the first image.
 24. The means of claim 23, further comprising: means for extracting a sub-image of the face within the frame; and means for possibly at least approximately matching the face with at least one other face in a database.
 25. The means of claim 24, further comprising: means for evaluating the matching the face with the at least one other face in the database; and means for determining a confidence level of recognition of the face within the frame.
 26. The means of claim 25, further comprising means for providing infrared lighting on the scene as needed for obtaining the first and second images.
 27. The means of claim 26, wherein: the lower reflective infrared band of light is approximately between 800 and 1400 nanometers; and the upper reflective infrared band of light approximately between 1400 and 2200 nanometers. 